1. Field of the Invention
The present invention relates to a high frequency low loss electrode for use in transmission lines and resonators operative in a microwave band and a millimeter wave band which are used mainly in radio communications, a transmission line, a high frequency resonator, a high frequency filter, an antenna sharing device, and communications equipment, each including the high frequency low loss electrode.
2. Description of the Related Art
Strip-type transmission lines and microstrip-type transmission lines, which can be easily produced and of which the size and weight can be reduced, are generally used in microwave IC""s and monolithic microwave IC""s operated at a high frequency. Resonators for such uses, in which the above-described lines have a length equal to a quarter-wavelength or a half-wavelength, or a circular resonator containing a circular conductor, are employed. The transmission loss of these lines and the unloaded Q of the resonators are determined mainly by the conductor loss. Accordingly, the performance of the microwave IC""s and the monolithic microwave IC""s depends on how much the conductor loss can be reduced.
These lines and resonators are formed with conductors with a high conductivity such as copper, gold, or the like. However, the conductivities of metals are inherent in the materials. There are limits to how much the loss can be reduced by selecting a metal with a high conductivity, and forming the metal into an electrode. Accordingly, great attention has been given to the fact that at the high frequency of a microwave or a millimeter wave, a current is concentrated at the surface of an electrode, due to the skin effect, and most of the loss occurs in the vicinity of the surface (hereinafter the xe2x80x9csurface portionxe2x80x9d) of the conductor.
It has been attempted to reduce the conductor loss from the standpoint of the structure of the electrode. For example, in Japanese Unexamined Patent Publication 8-321706, a structure is disclosed in which plural linear conductors with a constant width are arranged in parallel to the propagation direction at constant intervals to reduce the conductor loss. Moreover, in Japanese Unexamined Patent Publication 10-13112, a structure is disclosed in which the surface portion of an electrode are divided into plural parts, so that a current concentrated at the surface portion is dispersed to reduce the conductor loss.
However, the method in which the whole of an electrode is divided into plural conductors having an equal width as disclosed in Japanese Unexamined Patent Publication 8-321706 has the problem that the effective cross-sectional area of the electrode is decreased, so that the conductor loss cannot be effectively reduced.
The method in which the surface portion of the electrode is divided into plural sub-conductors having substantially the same width, as disclosed in Japanese Unexamined Patent Publication 10-13112, is effective to some degree in relaxing the current concentration and reducing the conductor loss. However, for modern high-frequency communications applications, further improvement is needed.
Accordingly, it is an object of the invention to provide a high frequency low loss electrode having reduced conductor loss.
It is another object of the present invention to provide a transmission line, a high frequency resonator, a high frequency filter, an antenna sharing device, and communications equipment, each having a low loss due to the use of the above-described high frequency low loss electrode.
The present invention has been achieved based on a finding that in an electrode having an end portion divided into plural sub-conductors, the conductor loss can be effectively reduced by setting the widths of the sub-conductors according to a predetermined principle.
According to the present invention, there is provided a first high frequency low loss electrode which comprises a main conductor, and at least two sub-conductors formed along a side of the main conductor, the sub-conductors being formed so that a sub-conductor thereof positioned nearer to the outside has a smaller width.
Preferably, in the first high frequency low loss electrode of the present invention, the sub-conductor positioned nearest to the outside of said sub-conductors has a width smaller than (xcfx80/2) times the skin depth xcex4 at an applied frequency. Consequently, an ineffective current flowing in the sub-conductor positioned nearest to the outside can be reduced. More preferably, to reduce the ineffective current flowing in the sub-conductor positioned nearest to the outside, the sub-conductor has a width smaller than (xcfx80/4) times the skin depth xcex4 at an applied frequency.
Still more preferably, in the first high frequency low loss electrode of the present invention, to reduce ineffective currents flowing in all the sub-conductors, all the sub-conductors have a width smaller than (xcfx80/2) times the skin depth xcex4 at an applied frequency.
More preferably, in the first high frequency low loss electrode of the present invention, the plural sub-conductors are formed so that a sub-conductor thereof positioned nearer to the outside is thinner, and thereby, the conductor loss can be reduced more effectively.
Moreover, in the first high frequency low loss electrode of the present invention, sub-dielectrics may be provided between the main conductor and the sub-conductor adjacent to the main conductor and between adjacent sub-conductors, respectively.
Also, preferably, in the first high frequency low loss electrode of the present invention, to cause currents to flow substantially in phase through the respective sub-conductors, the interval between the main conductor and the sub-conductor adjacent to the main conductor, and the intervals between adjacent sub-conductors, are formed so that an interval thereof positioned nearer to the outside is shorter, corresponding to the widths of the respective adjacent sub-conductors.
Still more preferably, in the first high frequency low loss electrode of the present invention, to cause currents to flow substantially in phase through the respective sub-conductors, the plural sub-dielectrics are formed so that a sub-dielectric thereof positioned nearer to the outside of the plural sub-dielectrics has a lower dielectric constant correspondingly to the widths of the respective adjacent sub-conductors.
Further, according to the present invention, there is provided a second high frequency low loss electrode which comprises a main conductor, and at least one sub-conductor formed along a side of the main conductor, said at least one sub-conductor having a width smaller than (xcfx80/2) times the skin depth xcex4 at an applied frequency. Consequently, in a sub-conductor of which the width is set at a value smaller than (xcfx80/2) times the skin depth xcex4 at an applied frequency, an ineffective current can be reduced, and the conductor loss can be effectively decreased.
More preferably, in the second high frequency low loss electrode of the present invention, said at least one sub-conductor has a width smaller than (xcfx80/4) times the skin depth xcex4 at an applied frequency.
Still more preferably, in the second high frequency low loss electrode of the present invention, the sub-conductor positioned nearest to the outside of the sub-conductors, and advantageously all of the sub-conductors, have a width smaller than (xcfx80/2) times the skin depth xcex4 at an applied frequency.
More preferably, in the second high frequency low loss electrode of the present invention, the sub-conductor positioned nearest to the outside of the sub-conductors, and advantageously all of the sub-conductors, have a width smaller than (xcfx80/4) times the skin depth xcex4 at an applied frequency.
In the second high frequency low loss electrode of the present invention, sub-dielectrics may be provided between the main conductor and the sub-conductor adjacent to the main conductor and between adjacent sub-conductors, respectively.
Preferably, in the first and second high frequency low loss electrodes according to the present invention, the main conductor is a thin-film multi-layer electrode comprising thin-film conductors and thin-film dielectrics laminated alternately.
Still more preferably, in the first and second high frequency low loss electrodes according to the present invention, at least one of the main conductors and the sub-conductors is made of a superconductor.
A first high frequency resonator according to the present invention includes the above-described first or second high frequency low loss electrode.
A high frequency transmission line according to the present invention includes the above-described first or second high frequency low loss electrode.
A second high frequency resonator according to the present invention includes the above-described high frequency transmission line of which the length is set at a quarter-wavelength or a half-wavelength, multiplied by an integer.
Further, a high frequency filter according to the present invention includes the above-described first or second high frequency resonator.
Moreover, an antenna sharing device according to the present invention includes the above-described high frequency filter.
Further, a communications device according to the present invention includes the above-described high frequency filter or antenna sharing device.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.